Adds Google OAuth2 "For Installed Apps" authentication.

Hi Anton,

I have got a lot of good use out of your code, so I thought I should feed back a contribution.

I have only about six months of Python experience, so I'll be very pleased if you spot improvements. For example, I wanted to put the test stuff in your test directory, but couldn't see how to set up the imports correctly.  How do you do it?  Also, I do not yet know how to make a 'pip' installable package, or work with 'Travis'.  I'll have to let you fix that up

As you will see, I have tried very hard to ensure that you have an absolute minimum of changes to review; it's almost 100% optional additions.

Sincere regards,
MHB

NOTE (2013/09/17)  
I have created a 6:30 minute video in two parts, that explains how to use the test script of my contribution.
Part 1 - http://www.youtube.com/watch?v=Ka-r5tXVW6I
Part 2 - http://www.youtube.com/watch?v=Dvr58B9klbY

NOTE (2013/09/27)
I have done a major cleanup.  There are now even fewer differences between your client.py and my altered one.